Misa Amane could also be an interesting addition. Her love for Light and her possession of a Death Note would bring a different dynamic. In the wizarding world, she might be attracted to the power and mystery of magic. Her presence could create a love triangle - like situation among some of the Hogwarts students, or she could be used by dark wizards who learn about her Death Note and try to control her.
Hope Mikaelson would be really interesting. She has such a powerful combination of magic and her werewolf/vampire heritage. In the Harry Potter world, she could be a real force to be reckoned with. She might be able to teach the Hogwarts students some new ways of using magic, or she could get involved in the fight against the Death Eaters and show them a whole new level of combat magic.
Joker from the Phantom Thieves would be really interesting. He has a mysterious and complex personality. If he were to cross over into the Death Note world, his skills in manipulating situations and people could put him in direct conflict with Light. He might be able to outwit Light in ways that others couldn't, and his sense of justice, which is more about reforming society on a personal level, would contrast greatly with Light's more extreme views.
I think Harry Potter himself would be really interesting. Imagine him interacting with a character like Naruto. Harry has his own struggles with being the 'chosen one' in his world, and Naruto has a similar burden. They could share their experiences, and maybe Harry could learn some of Naruto's ninja techniques and apply them to his magic. It would be a really cool combination of two very different but also similar 'chosen one' characters.
Dumbledore could be an amazing crossover character. His vast wisdom and powerful magic would make him a force to be reckoned with in the Overlord world. He could potentially see the good in some of the 'evil' characters and try to guide them towards a better path. His presence would also shake up the power balance in the Overlord world as different factions would either fear or seek to ally with him.
Touka Kirishima could also be a fascinating crossover character. She's a strong - willed and capable ghoul in Tokyo Ghoul. If she got her hands on the Death Note, she might use it to protect her friends and the ghoul community in a more discreet way compared to her usual more direct combat methods. It would show a different side of her character, as she'd have to be more calculating and less impulsive.
I think Gandalf would be a very interesting character to cross over. He has so much wisdom and power in LOTR. In the Harry Potter world, he could share his knowledge of different magics and perhaps help Dumbledore in his fight against Voldemort. He could also introduce the concept of the greater good in a more profound way to the young wizards and witches.
Definitely Doctor Strange. He already has a deep understanding of magic in the Avengers universe. In the Harry Potter world, he could exchange knowledge with the wizards, learn new spells, and maybe even help improve the defenses of Hogwarts. He could also have some really cool battles against the dark wizards using his unique combination of magic and martial arts.
Hermione Granger would be really interesting. She could bring her intelligence and magic skills to the OITNB world. She might start a library or a study group for the inmates, teaching them about magic and also helping them with their own educational pursuits. Her strong - willed and determined nature would make her a force to be reckoned with in the prison environment.
Dumbledore could be an extremely interesting crossover character. His wisdom and powerful magic would make him a force to be reckoned with in the Kantai Collection world. He could potentially teach the shipgirls about different forms of magic, and his ability to see the bigger picture would be useful in strategizing against the Abyssal threats.
Definitely Doctor Octopus. Imagine him getting his hands on some magic wands and using his mechanical arms to perform complex spells. He could be a really formidable villain in the wizarding world, and it would be exciting to see how Harry Potter and Spiderman would team up to stop him.